When Cassini first arrived, the north pole was in darkness; it was winter in the planet's 29-Earth-year annual cycle.

Now it has taken some of its first sunlit images of the pole, which has not been seen since the Voyager 2 craft last sent pictures on its fly-by in 1981.

Andrew Ingersoll, a member of the Cassini team based at the California Institute of Technology in California, US, said: "We did a double take when we saw this vortex because it looks so much like a hurricane on Earth."

"But there it is at Saturn, on a much larger scale, and it is somehow getting by on the small amounts of water vapour in Saturn's hydrogen atmosphere."

The team believes the hurricane to be "stuck" at the pole, forced northward by winds in the same way hurricanes tend to move north on Earth.